I. Field of the Invention
The present invention relates to radio frequency signal communications. More particularly, the present invention relates to a novel and improved method for performing power control.
II. Description of the Related Art
The IS-95 Over-the-Air (OTA) Interface Standard defines a set of RF signal modulation procedures for implementing a digital cellular telephone system. The IS-95 Standard, and its derivatives, such as IS-95A and ANSI J-STD-008 (referred to collectively as the IS-95 Standard), are promulgated by the Telecommunications Industry Association (TIA) to insure the operability between telecommunications equipment manufactured by different vendors.
The IS-95 Standard has received enthusiastic reception because it uses the available RF bandwidth more efficiently than previously existing cellular telephone technologies. This increased efficiency is provided by using Code Division Multiple Access (CDMA) signal processing techniques in combination with extensive transmit power control to increase the frequency reuse of a cellular telephone system.
FIG. 1 illustrates a highly simplified digital cellular telephone system configured in a manner consistent with the use of IS-95. During operation, telephone calls and other communications are conducted by exchanging data between subscriber units 1 (generally cellular telephones) and base stations 2 using RF signals. Typically, communications are further conducted via wireline connections from base stations 2 through base station controllers (BSC) 4 and mobile switching center (MSC) 6 to either public switch telephone network (PSTN) 8, or to another subscriber unit 1. BSC's 4 and MSC 6 typically provide mobility control, call processing, and call routing functionality.
The RF signal transmitted from a base station 2 to a set of subscriber units 1 is referred to as the forward link signal, and the RF signal transmitted from a subscriber unit 1 to a base station 2 is referred to as the reverse link signal. The IS-95 Standard calls for subscriber units 1 to provide telecommunications service by transmitting user data such as digitized voice data via the reverse link signal. The reverse link signal is comprised of a single traffic channel, and therefore is often referred to as a "non-coherent" signal because it does not include a pilot channel.
Within the reverse link signal, user data is transmitted at a maximum data rate of 8.6 or 13.35 kbps, depending on which rate set from a set of rate sets provided by IS-95 is selected. The use of a single channel, non-coherent, reverse link signal simplifies the implementation of an IS-95 cellular telephone system by eliminating the need for synchronization between a set of subscriber units 1 communicating with a single base station 2.
As mentioned above, IS-95 incorporates extensive transmit power in order to more efficiently utilize the available RF bandwidth. In accordance with IS-95, this power control is performed by measuring the strength or quality of the reverse link traffic channel when received at the base station and generating a power control command based on that measurement. The power control command is transmitted to the subscriber unit via the forward link signal.
The subscriber unit responds to the power control command by increasing or decreasing the transmit power of the reverse link signal based on the power control command. This power control adjustment is performed repeatedly at rates on the order of 800 times per second in order to maintain the reverse link signal transmit power at the minimum necessary to conduct communications. Additionally, IS-95 also calls for transmit duty cycle of the reverse link signal to be adjusted in response to changes in voice activity in 20 millisecond increments. Thus, when the transmit duty cycle is lowered, the signal is transmitted at either a set point, or the signal is gated and not transmitted at all. During periods when the reverse link signal is gated, the base station generates incorrect power control increase commands because the reverse link signal is not detected. The subscriber unit can ignore these false increase commands, however, because it knows when the reverse link signal was and was not transmitted, and therefore when the false increase commands are generated.
To satisfy the ever increasing demand to transmit digital data created by networking technologies such as the worldwide web, a higher rate and more complex transmission system incorporating a multi-channel, coherent, reverse link signal is provided in co-pending U.S. patent application Ser. No. 08/654,443 entitled "High Data Rate CDMA Wireless Communications System" filed on May 28, 1996, assigned to the assignee of the present invention and incorporated herein by reference ('443 application). In particular, the above referenced patent application describes a reverse link signal including at least one traffic channel, a power control channel, and a pilot channel.
The use of a multi-channel reverse link signal provides various advantages including increased flexibility because different types of data may be transmitted simultaneously over the set of channels. Additionally, providing a pilot channel in the multi-channel reverse link signal facilitates coherent processing of the reverse link signal which improves processing performance.
It is also desirable to perform reverse link power control for the high speed link described in the above referenced patent application in order to continue to make efficient use of the available RF bandwidth. In one implementation of the high data rate system described in the above referenced patent application, however, the reverse link signal is transmitted continuously, with the transmit power of the traffic channel being adjusted incrementally in 20 millisecond increments in response to changes in the data rate, typically brought about by changes in the voice activity. That is, the traffic channel is transmitted at a reduced power level rather than at a reduced duty cycle during each 20 ms increment when the data rate decreases. Typically, the transmit power could be one of four levels that can be used for one of four voice activity increments, however, any number of transmit power levels can be used.
Thus, the transmit power for the high data rate system varies over wider range values than for IS-95, which is either transmitted at the set point or completely gated. Also, the transmit power in the higher rate system can remain low for a longer period of time than for IS-95, since IS-95 requires at least some set point transmissions during each frame, while no set point transmissions may occur for several frames for the higher rate system if the data rate remains low. Since the system receiving the high rate link will not know whether this reduction is due to increased distance, or simply a result of the reduced data rate, it will be difficult to determine the appropriate power control command to transmit. Since it is nonetheless desirable to perform reverse link power control in this high rate system, a new method for reverse link power control is required.